源码看这里

Promise.prototype.then = function(onFulfilled, onRejected) {
  if (this.constructor !== Promise) {
    return safeThen(this, onFulfilled, onRejected);
  }
  var res = new Promise(noop);
  handle(this, new Handler(onFulfilled, onRejected, res));
  return res;
};

function safeThen(self, onFulfilled, onRejected) {
  return new self.constructor(function (resolve, reject) {
    // 以下不是很理解
    var res = new Promise(noop);
    res.then(resolve, reject);
    handle(self, new Handler(onFulfilled, onRejected, res));
  });
}